In multiple-input multiple-output (MIMO) communications systems, antennas may be used on both a base station and on a mobile device to exploit a phenomenon known as multipath propagation to achieve higher data rates. In general, MIMO communications systems simultaneously send and receive multiple data signals over each radio channel. The multipath propagation phenomenon is the result of environmental factors that influence the data signals as they travel between the base station and the mobile device, including, for example, ionospheric reflection and refraction, atmospheric ducting, reflection from terrestrial objects and reflection from bodies of water. Because of these factors, the data signals experience multipath interference that results in constructive interference, destructive interference, or fading, and phase shifting of the data signals. The base stations and mobile devices of MIMO communications systems may each have an advanced antenna. For example, an advanced antenna may be configured as an antenna array or multiple radiating antenna elements configured on a single antenna, wherein each antenna (in an antenna array) and radiating antenna element (for a single antenna) is individually logically controllable to collectively form beams. A complex weight pattern is introduced to the advanced antenna in a time domain signal to form a beam.
MIMO communications systems require testing. A typical MIMO test system for testing a device under test (DUT) includes an anechoic chamber, the DUT in the anechoic chamber, a test system computer, and various electrical cables for interconnecting components. An anechoic chamber is a chamber designed to absorb reflections of sound (or electromagnetic) waves. The test system computer may be a network emulator that emulates a base station when the DUT is a mobile device, and a user equipment (UE) device emulator that emulates a mobile device when the DUT is a base station. During over the air (OTA) testing, the test system computer receives information from the DUT that the test system computer processes to evaluate the transmit and/or receive capabilities of the DUT as the DUT is subject to the over the air testing. The over the air testing may involve controlling an advanced antenna of the DUT, for example, to communicate by selectively controlling each antenna (in an antenna array) or radiating antenna element (for a single antenna) to form beams. Beam characteristics (e.g., total transmit power, error vector magnitude of modulation formats, antenna radiation pattern) are compared against expectations to measure whether the advanced antenna of the DUT works properly.
Millimeter wave (MMW, or mmWave) is also known as extremely high frequency (EHF) or very high frequency (VHF), and is the band of spectrum between 30 gigahertz (GHz) and 300 gigahertz. Centimeter wave (CMW, or cmWave) is also known as super high frequency (SHF), and is the band of spectrum between 3 gigahertz and 30 gigahertz. This entire band of spectrum is within the radio frequency spectrum, which extends between 3 kilohertz to 300 gigahertz. The next telecommunications standards (i.e., beyond the current 4G LTE telecommunications standards) are referred to as 5th generation (5G) telecommunications standards, and may standardize communications systems that use millimeter wave and/or centimeter wave technology. Telecommunications standards such as 5G and 4G LTE are examples of radio access protocols and standards that define a timing scheme including the order and timing of communications actions for communications in a defined portion of the radio frequency spectrum.
Electromagnetic waves with high frequencies in the area between 3 gigahertz and 300 gigahertz experience significant propagation loss (path loss) and diffraction. To overcome this high path loss and provide sufficient coverage to users on the edge of coverage in/of a cell using these high frequencies, beamformed signal transmissions use high directivity in the desired direction of transmission. This means that the radiation patterns of beams become narrower, which in turn may increase the number of beams or types of beams. Beamformed signal transmissions with high directivity are used by a network emulator that exercises a DUT with different signals each with a different angle of arrival (AoA). The testing setup for such over the air testing traditionally involves the network emulator communicating with the DUT via several fixed antennas radiating in a fixed set of directions in the anechoic chamber. Initially, for a communications system using high frequencies in the area between 3 gigahertz and 300 gigahertz, the network emulator performs beam-sweeping during initial access to a network by the DUT to transmit broadcast and synchronization signals in multiple spatial directions following a predefined pattern. The DUT will eventually determine an optimal spatial direction from the network emulator and use this information to determine the direction for transmissions from the DUT to the network emulator. Once this initial beam determination has been performed, subsequent data transmissions between the network emulator and the DUT occur over a single spatial direction pointed to the DUT.
In the testing exercises, the test purpose is to verify that the DUT correctly identifies the different signals, determines and selects the best angle of arrival, transmits to the network emulator accordingly and keeps track of how the angle of arrival changes to adapt to varying channel conditions. After the initial access process, beam-sweeping is repeatedly performed by the network emulator using different beams in a predefined pattern.
In conventional testing solutions which use discrete angles of arrival, the network emulator needs as many transmitters and receivers as different spatial directions and polarizations that will be used since there is no a-priori knowledge of the DUT transmission direction or polarization. For example, a setup may involve eight (8) different signals that can be transmitted in four different spatial directions by using two (2) polarizations, horizontal (H) and vertical (V) in each direction. These traditional solutions require the network emulator to have as many receivers as transmitters.